Frequency mixers, generally also called frequency converters, form important components in a multiplicity of different applications pertaining to radiofrequency technology. They generally serve to convert a signal having a first center frequency to an output signal having a second frequency with the aid of a local oscillator signal. In this case a distinction is made between mixers in which the frequency of the output signal is greater than the frequency of the input signal and mixers in which the frequency of the output signal is less than the input signal frequency. The former are also referred to as up-converters, and the latter as down-converters.
In the case of down-converters, a distinction is additionally made between frequency converters which convert the input signal to a low intermediate frequency or to an intermediate frequency of 0 HZ. Conversion to an intermediate frequency of 0 HZ additionally has the advantage that the converted signal can be subjected to further processing directly after suitable filtering. Modern telecommunications frequency converters which effect conversion to an intermediate frequency of 0 HZ are best suited to realizing customers' requirements for low current consumption, small chip area and hence low price. In the meantime, frequency converters of this type are being implemented as integrated circuits in semiconductor bodies using purely CMOS technology (complementary MOS technology). The requirements made of the transfer response, the signal/noise ratio and the current consumption of the frequency converters of this type are rising at the same time.
A significant aspect in the use of mixers with conversion to the intermediate frequency 0 HZ using CMOS technology is so-called flicker noise, which is brought about by the defects in the gate material of the MOS transistors within the mixer and leads to a reduction of the signal/noise ratio. Flicker noise usually constitutes a dominant noise source in mixers having primarily a small signal bandwidth which are formed using field-effect transistors. Further flicker noise sources situated upstream of the actual mixer cell within the mixer are converted to higher frequencies and mainly do not cause any impairment of the signal/noise ratio.
Flicker noise is proportional to the current flowing through the transistor and proportional to the frequency of the local oscillator signal. A reduction of the flicker noise of the frequency converter on the output side can be achieved for example by increasing channel length of the field-effect transistors used. However, this leads to an increased loading on the driver circuits for providing the local oscillator signal and thus to a reduction of the changeover speed. Since flicker noise principally becomes apparent at the changeover instant at the output of the frequency converter, the overall performance may be impaired as a result.
FIG. 7 shows a known down-converter realized as a slightly modified Gilbert mixer. The heart of the frequency converter is a mixer cell 1, which comprises four field-effect transistors T5 to T8 as illustrated here. The radiofrequency signal RF+, RF− to be converted, which is present as a differential-mode signal in the present case is converted into a current signal by means of transistors T1 to T2 operating as a voltage-current converter and is fed to the actual mixer cell 1. On the output side, the mixer cell is connected to the output taps via two field-effect transistors T13 and T14 acting as a cascade. The frequency-converted current signal is converted into a voltage signal again by means of the resistors R1 and R2.
In order to reduce the flicker noise in the mixer cells, in this known present case, part of the DC current is extracted via the additional current sources I1, I2. This decreases the direct current through the mixer cell 1 and thus the flicker noise as well. A reduction of the DC current through the current sources I1 to I4 is effected only as long as the linearity requirement of the mixer cell 1 can be complied with.
According to the Inventor's knowledge, a further possibility for reducing flicker noise comprises implanting fluorine ions into the field-effect transistors of the mixer cell 1. This technology option relates primarily to production, where it causes additional process steps and hence an increase in the production costs.